I couldn't miss the chance to practice my drawing!
Battle in the Big Keep is a Trial in Final Fantasy XIV: A Realm Reborn, introduced in patch 2.5, Before the Fall. It sees players battle against the Gilgamesh and Enkidu as part of the Hildibrand Manderville questline.
How many realms has he traversed, and how many rivals has he felled? Gilgamesh journeys ever onward in his unending quest for the strongest of weapons. But the road is long and lonesome, and a painted rooster—loyal and noble as the bird may be—can bring a man only so much comfort in the darkest and most trying of times. So it was that Gilgamesh turned to the heavens, pleading to be reunited with a long-lost companion. Be it by fortune or fate, his prayers were heard, and the stage for the final confrontation is set. To the victor goes the Treaty-Blade!Duty Finder description
Behind the scenes
The fight taking place in Amdapor Keeper is probably a reference to the fight against him in Castle Exdeath in Final Fantasy V, where he first morphs.
Many of Gilgamesh's lines, including the achivement "Enough Expository Banter", are based on his dialogue from Final Fantasy V.
Missile and Jump are attacks introduced to Gilgamesh in Final Fantasy V. Masamune is one of his summon abilities in Final Fantasy VIII. Bitter End is an ability used by Gilgamesh in Final Fantasy XII. Whirlwind Slash is one of Gilgamesh's attacks in Dissidia 012 Final Fantasy. Sword Dance is first used by Gilgamesh in Final Fantasy VI Advance.